It is well-known among persons skilled in the art that sludge from municipal purification plants constitutes a large waste problem in most industrialized countries. The sludge can e.g. be generated at a chemical treatment of sewage water which subsequently is dewatered. The dewatered sludge typically consists of 70–80% water, 10–15% organic material and 10–15% mineral components.
Sludge waste can in either wet or dried form be spread as fertilizer on farmland. The content of the sludge of e.g. heavy metals and iron and aluminum phosphates of low solubility cannot be utilized by the crops and there is therefore a risk of these substances percolating into the ground water or destroying the soil structure.
Alternatively, dried sludge waste can be deposited in very large landfills. The space requirements to the landfills mean that such sites must be open. When the sludge is exposed to precipitation, a possible content of heavy metals and trace elements will be leached out and pollute the surrounding environment.
An often used method for disposing of sludge waste is to incinerate the sludge. Hereby, an ash is produced that subsequently must be deposited. The above-mentioned heavy metals and iron and aluminum phosphates are now merely to be found in the ash, and the ash will at depositing result in the same leaching-out and percolation problems as mentioned above. To this should be added that the caloric value of dried sludge is very small compared to the caloric value of traditional combustibles. As an example of this, it can be mentioned that dried sludge has a caloric value of 12–13 MJ/kg which is about half of that of wood. The small caloric value therefore means that dried sludge is used very occasionally if ever as an energy source.
The industry produces large amounts of waste products that only very rarely can be reused and therefore also constitute a significant and costly depositing problem. By reusing the above waste products, the growing and therefore increasingly costly need for depositing areas can be reduced.
There is therefore a need for reusing in an economically advantageous way a wide range of waste products in order to thereby reduce the need and requirements to the depositing areas and without at the same time producing deposit material containing environmental harmful and health hazardous substances. The present invention now provides solutions to these problems.